Our goal is to understand the molecular basis for the specificity of interactions between neurons in the nervous system. The application of hybridoma technology to analyze the molecular organization of neurons is currently one of the most promising approaches to this goal. The leech is particularly suited to this study because of the simple anatomy of it's nervous system, the detailed characterization of many identified neurons, our knowledge of synaptic circuits regulating rythmic behaviours and the analysis of neural development at the single cell level. We have developed an array of monoclonal antibodies which identify antigens present in individual neurons and groups of neurons. These antigens are found in electrophysiologically defined subsets of neurons. No special class of molecule distinguishes one neuron from another suggesting that understanding neuronal diversity will require knowledge of several distinct types of molecule. We have focussed on extending our observation that sterotypically located fascicles of axons carry specific surface glyco-protein antigens in the adult and embryo. The experiments in this proposal are designed to use monoclonal antibodies to extend our knowledge of axon organization so that we gain insight into general rules which govern axon morphology and consequently synaptic interactions at the molecular level.